Generally, a driver of a liquid crystal display (LCD) includes both a digital part and an analogue part. The digital part controls color data signals which are outputted from a computer, such as a personal computer, and a graphic controller, and the analogue part selects a gray voltage corresponding to a gray of each pixel in accordance with the color data signals and applies the gray voltage to the LCD panel.
The color data signal is a square wave swinging between 5V and 0V. In typical twisted nematic (TN) mode of an LCD, the gray voltages are the voltages generated by dividing a voltage of 4V or 5V since the gray of pixel is displayed within a range of 4V or 5V. Therefore, a power supply in the LCD is a circuit for supplying 5V of single power voltage.
However, as the technology improves and the resolution of LCD increases, color data signals will be applied to more pixels. Therefore, the frequency of the color data signal will increase, thereby causing an electromagnetic field (EMF) in the digital part. While an increase in the resolution of LCDs is beneficial, the increase in EMF is not beneficial. In fact, generally, EMF is considered harmful to the human body at a high frequency and large voltage. Therefore, it would considered most beneficial to instead of using 5V as a driving voltage to use a lower driving voltage, such as 3.3V, to reduce the EMF harmful effects.
To overcome the above-mentioned problem, the digital part in a conventional LCD is designed to use 3.3V of a driving voltage, so the power supply supplies 5V for the analogue part and 3.3V for the digital part.
FIG. 1 shows a conventional power supply supplying dual power voltages of 3.3V and 5V. Referring to FIG. 1, a source voltage Vcc, which is a digital voltage of 3.3V, is directly outputted as the digital part voltage Vd (hereinafter digital voltage). A DC/DC converter 10 generates 5V after receiving the source voltage Vcc, and the 5V is outputted as the analogue part voltage Va (hereinafter analogue voltage).
The LCD using the above mentioned power supply requires a level shifter for shifting a voltage level. One example is the case that the 3.3V should be converted to 5V for applying the LCD panel. In detail, a common electrode voltage Vcom swinging between 5V and 0V is made by using a reverse signal swinging between 3.3V and 0V.
FIG. 2A shows a conventional level shifter, and FIG. 2B shows wave forms of input and output signals of the level shifter shown in FIG. 2A. Referring to FIG. 2A, the conventional level shifter comprises a CMOS inverter including a PMOS transistor M1 and an NMOS transistor M2. The PMOS transistor M1 receives the analogue voltage Va through its source, and the source of the NMOS transistor M2 is connected to the ground GND. The inverter receives an input signal IN swinging between the digital voltage Vd and the ground GND level through its common gate, and outputs an output signal OUT through its common drain.
Generally, the threshold voltage Vthp of the PMOS transistor M1 has a negative level, while the threshold voltage Vthn of the NMOS transistor M2 having a positive level, and the analogue voltage Va is greater than the digital voltage Vd. Therefore, the relation of each voltage can be expressed as: EQU Vthp&lt;0&lt;Vthn&lt;Vd&lt;Va.
According to the typical characteristics of a transistor, the PMOS transistor M1 is turned on when the gate-source voltage is less than its own threshold voltage Vthp, and the NMOS transistor M2 is turned on when the gate-source voltage is greater than its own threshold voltage Vthn.
If the input signal IN is the ground level voltage, the PMOS transistor M1 will be turned on, the NMOS transistor M2 will be turned off, and the output signal OUT keeps the analogue voltage Va level as shown in FIG. 2B.
If the input signal IN is the digital voltage Vd, the NMOS transistor M2 will be turned on since the gate-source voltage of the transistor M2 is greater than its own threshold voltage Vthn. Therefore, the output signal OUT keeps the ground GND level.
However, if the gate-source voltage of the PMOS transistor M1 is less than its own threshold voltage Vthp when the input signal IN is the digital voltage Vd, the PMOS transistor M1 will be turned on, so an unneeded and unwanted current path will be formed to lead from the analogue voltage Va through the PMOS transistor M1 and the NMOS transistor M2 to the ground.
Therefore, the unwanted current path causes the power consumption of the LCD to increase by a substantial and therefore an undesirable amount.